1. Field of the Invention
The present invention relates to a heat treatment method and a heat treatment apparatus for heating a thin plate-like precision electronic substrate such as a semiconductor wafer and a glass substrate for a liquid crystal display device (hereinafter referred to simply as a “substrate”) by irradiating the substrate with light, to thereby activate the impurities.
2. Description of the Background Art
In the process of manufacturing a semiconductor device, impurity doping is an essential step for forming a pn junction in a semiconductor wafer. At present, it is common practice to perform impurity doping by an ion implantation process and a subsequent annealing process. The ion implantation process is a technique for causing ions of impurity elements such as boron (B), arsenic (As) and phosphorus (P) to collide against the semiconductor wafer with high acceleration voltage, thereby physically implanting the impurities into the semiconductor wafer. The implanted impurities are activated by the subsequent annealing process. When annealing time in this annealing process is approximately several seconds or longer, the implanted impurities are deeply diffused by heat. This results in a junction depth much greater than a required depth, which might constitute a hindrance to good device formation.
In recent years, attention has been given to flash lamp annealing (FLA) that is an annealing technique for heating a semiconductor wafer in an extremely short time. The flash lamp annealing is a heat treatment technique in which xenon flash lamps (the term “flash lamp” as used hereinafter refers to a “xenon flash lamp”) are used to irradiate the surface of a semiconductor wafer with a flash of light, thereby raising the temperature of only the surface of the semiconductor wafer doped with impurities in an extremely short time (several milliseconds or less).
The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamps is shorter than that of light emitted from conventional halogen lamps, and approximately coincides with a fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with a flash of light emitted from the xenon flash lamps, the temperature of the semiconductor wafer can be raised rapidly, with only a small amount of light transmitted through the semiconductor wafer. Also, it has turned out that flash irradiation, that is, the irradiation of a semiconductor wafer with a flash of light in an extremely short time of several milliseconds or less allows a selective temperature rise only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short time with the xenon flash lamps allows only the activation of impurities to be achieved without deep diffusion of the impurities.
Heat treatment apparatuses which employ such xenon flash lamps are disclosed in U.S. Pat. Nos. 4,649,261 and 6,849,831 in which pulsed light emitting lamps such as flash lamps are disposed on the front surface side of a semiconductor wafer and lamps that stay lit continuously such as halogen lamps are disposed on the back surface side thereof so that a desired heat treatment is performed using a combination of these lamps. In the heat treatment apparatuses disclosed in U.S. Pat. Nos. 4,649,261 and 6,849,831, a semiconductor wafer is preheated to a certain degree of temperature by the halogen lamps and the like, and is then raised in temperature to a desired treatment temperature by pulse heating from the flash lamps.
However, a heat treatment apparatus employing such a xenon flash lamp, which momentarily irradiates the semiconductor wafer with light having ultrahigh energy, rapidly raises the temperature of the front surface of the semiconductor wafer for a very short period of time. As a result, it has been found that process-induced damage resulting from the abrupt temperature rise occurs to exert an adverse influence on characteristics of the semiconductor device, so that a desired reliability lifetime cannot be obtained.
Also, the implantation of high-energy ions by the ion implantation process results in the induction of a large number of defects in silicon crystals of the semiconductor wafer. Such defects are prone to be induced in positions slightly deeper than an ion-implanted layer. During the annealing process subsequent to the ion implantation, it is desirable to perform the recovery of the induced defects as well as the activation of impurities. For such recovery of the defects, the time for annealing process may be made longer. This, however, presents a problem such that the impurities implanted as mentioned above are diffused more deeply than are required.
To solve such a problem, a flash lamp annealing technique which performs additional irradiation with light with a relatively low emission output after a peak of the emission output is passed is disclosed in U.S. Patent Application Publication No. 2009/0263112. According to the technique disclosed in U.S. Patent Application Publication No. 2009/0263112, the temperature of the front surface of a semiconductor wafer is raised to a treatment temperature, and is thereafter maintained at the treatment temperature for approximately several milliseconds or more by the additional irradiation with light. This allows the heating of the semiconductor wafer in a position slightly deeper than the front surface to some extent, thereby accomplishing not only the activation of the impurities but also the recovery of the induced crystal defects.
In flash lamp annealing, however, when the temperature of the front surface of the semiconductor wafer is raised to the treatment temperature and is thereafter maintained at the treatment temperature, there is a danger that the frequency of occurrence of wafer cracking increases. This is considered to result from the following reason. In the flash lamp annealing in which the front surface of a semiconductor wafer is heated by irradiation for an extremely short time, there inevitably arises a difference in temperature between the front and back surfaces of the semiconductor wafer. Maintaining the temperature of the front surface of the semiconductor wafer at the treatment temperature increases the time period over which there is a large temperature difference between the front and back surfaces, so that stresses resulting from a difference in thermal expansion between the front and back surfaces are concentrated on the back surface of the semiconductor wafer.